The presence of two adjacent carbon-carbon and carbon-oxygen double bonds confers a very high reactivity on dimethylketene. It is advantageous to selectively direct the opening of one or other of the double bonds in order to promote uniform polymerization of the monomer units (A), resulting in polymers with β-ketone structures (PolyA), or uniform polymerization of the monomer units (B), resulting in polymers with structures of vinyl polyacetal type (PolyB), indeed even in the alternating addition of the (A) and (B) units, resulting in a vinyl polyester (PolyAB).

The first polymerization of DMK was described by H. Staudinger in 1925. He carried out the polymerization at a temperature of −80° C. to 0° C. in the presence of trimethylamine as catalyst. The product obtained was described as an amorphous and noncrystallizable cyclic trimeric compound comprising 3 units of monomer (A) and 2 units of monomer (B).
Patent GB-893 308 discloses the synthesis of a crystalline polymer resulting from the polymerization of (A) and (B) units at a temperature of between −80° C. and −20° C. with aluminum-based catalysts (AlCl2(C2H5), AlCl(C2H5)2, Al(C6H5)Cl2, Al(OC3H7)Br2, AlBr3, AlCl8.O(C2H5)2 or AlEt3), a beryllium-based catalyst (Be(C2H5)2) or a zinc-based catalyst (Zn(C4H9)2).
Patent GB-987 370 relates to an improvement to the preceding patent. It provides a process for the synthesis of PDMK comprising more than 93% by weight of β-ketone unit by carrying out the synthesis in a solvent having a high dielectric constant and in the presence of AlBr3 or of AlCl2Et. The solvents used are nitrobenzene, toluene, dichloromethane, 1,1,1,2-tetrachloroethane and 1,1-dichloroethylene.
The state of the art in the field of the cationic polymerization of dimethylketene (DMK) does not report the use, on the one hand, of an initiator (I) or of a cocatalyst (CoK). The catalyst generally employed is a Lewis acid suspended alone in a solvent or a solvent mixture. In a few cases, the Lewis acid is dissolved in a polar solvent or a mixture of solvents, at least one of which is polar.
However, while these processes using only a Lewis acid as catalyst make it possible to obtain a β-ketone polymer with a high selectivity, the synthetic yields are too low to envisage an industrial operation (yield <65%). This is because, in such systems, two forms of initiation are superimposed:                Initiation due to the reaction between the traces of water and the Lewis acid, releasing an acidic proton, the true initiator of the cationic polymerization of DMK.        Initiation due to the self-dissociation of the Lewis acid, generating two ionic entities with opposite charges by the BIE (Binary Ionogenic Equilibria) mechanism. In this case, a zwitterion entity, which is partly the cause of the formation of the trimers, is generated. The use of polar solvents is then essential to separate the charges and to limit the formation of trimers but these polar solvents (nitrobenzene, nitro derivatives, chloroform), because of their toxicity, restrict large-scale production of these polymers. It is also important to note two additional difficulties in this chemistry: the toxicity of DMK and its propensity to generate explosive peroxides.        
The Applicant Company has now found that, depending on the operating conditions for the polymerization of DMK, it is possible to selectively direct the polymerization toward the formation of a polymer of β-ketone structure with very good yields, >65%, and in the presence of conventional inexpensive solvents which do not exhibit the degrees of dangerousness of the solvents mentioned above. Furthermore, for better effectiveness and better reproducibility, the Applicant Company has adopted a position in cationic catalysis requiring a catalyst which is soluble in the solvent used. It has also disposed of the problems of formation of peroxides, in order to make possible safe production of PDMK, and problems of transfer reactions which interfere with the results and bring about the formation of chains of low molecular mass. These various parameters make possible large-scale synthesis of PDMK.